Disposable rubber gloves used in daily life such as housework, food industry, electronics industry, medical field, etc. are made by dip-molding of natural rubber or a carbonic acid-modified nitrile-based copolymer latex. In recent years, due to allergy issues and unstable supply/demand problems of natural rubber, carbonic acid-modified nitrile-based gloves are in the spotlight on the disposable glove market.
Currently, nitrile gloves companies are constantly striving to reduce costs as well as enhance quality as competition between companies becomes more intense.
Typical markets for nitrile rubber gloves are the thick gloves represented by industrial applications and the thin glove market represented by medical applications, and the approach is different for each application area for cost reduction.
In general, nitrile rubber gloves are made by adding inorganic pigments such as calcium carbonate to latex compositions for dip-molding to keep the gloves thick while lowering the manufacturing cost. However, since the latex composition for dip-molding is low in stability against calcium cations in calcium carbonate, calcium carbonate coagulates to generate a large amount of coagulation (agglutination, flocculation, agglomeration or coalescence).
The coagulated product appears as a pinhole after the dip-molding step to increase the defective rate of the glove, and if the generation of the coagulated product is serious, the manufacture of the glove itself becomes impossible. As a result, the amount of calcium carbonate used for cost reduction is extremely limited.
On the other hand, the nitrile glove maker produces a thinner glove than the thicker glove, thus reducing the cost by reducing the weight of the glove.
Thin gloves can be manufactured by very low concentrations of latex, which causes syneresis, severe depletion of water during the formation of glove films during dip-molding processes. This generates a coagulum of the latex composition to produce multiple pinholes of the final glove, thereby increasing the rejection rate of the glove.
In general, the syneresis tends to be improved by increasing the particle size of the latex, in this case, gloves made of latex of large particles are accompanied by a problem of lowering tensile strength. Therefore, glove makers are continuously interested in carbonic acid-modified nitrile-based latex which does not deteriorate the tensile strength of the final glove while delaying the syneresis during dip-molding even if the glove is made thin.
The present applicant has proposed a technique using a carbonic acid-modified nitrile-based copolymer latex and starch in Korean Patent Publication No. 2014-0113290. In the above patent, the use of starch solves the problem of the syneresis and the tensile strength to some extent, and it is possible to reduce the stickiness during the production of the dip-molded article.
However, this patent merely mixes latex and starch with latex compositions for dip-molding and does not suggest the use of inorganic pigments such as calcium carbonate.
In addition, when calcium carbonate is used for cost reduction as mentioned above, there is a problem that dispersion stability is low when mixed with latex. Therefore, even if starch is used, the dispersion stability according to the use of calcium carbonate and the problem of the defect of the dip-molded article are conventionally retained.
Therefore, even if calcium carbonate is used to reduce the cost regardless of the thickness of the glove, it is urgent to develop a technique for improving the dispersion stability of latex as well as physical properties such as syneresis and tensile strength.